Fibromyalgia (FM) is a chronic musculoskeletal pain syndrome that causes substantial physical and psychological impairment and costs over $25 billion annually. Current pharmacological therapies may be expensive, cause serious adverse effects, and fail to effectively improve pain and function. Thus, finding new and effective non-pharmacological treatments for FM patients is urgently needed. Previous research has demonstrated that Tai Chi, a traditional Chinese mind-body exercise, can improve both physical and mental health for FM patients. However, no clinical trials have demonstrated its clinical effectiveness compared to standard aerobic exercise therapy, or determined its optimum dose (frequency or duration) as a treatment for FM. We therefore propose to conduct the first comparative effectiveness trial of Tai Chi vs. aerobic exercise (a recommended component of the current standard of care) and to evaluate effectiveness under different Tai Chi dosing schedules in a large FM population. We aim to (1) demonstrate that, compared to aerobic exercise, Tai Chi is a more effective intervention for managing the pain and improving the functional limitations that impact quality of life for FM patients, and 2) determine the optimal frequency and duration of a supervised Tai Chi intervention in relation to short and long-term effectiveness. To achieve this goal, we will conduct a single-blind, randomized controlled trial of Tai Chi vs. aerobic exercise in 216 patients who meet the American College of Rheumatology criteria for FM. Patients will be randomized to one of four Tai Chi intervention groups: 12 or 24 weeks of supervised Tai Chi given once or twice per week, or a supervised aerobic exercise control (2x/week for 24 weeks). All groups will have a 52-week follow-up. The primary outcome will be the FM Impact Questionnaire total score at 24 weeks. Secondary outcomes include the measures of widespread pain, functional performance, psychological functioning, self-efficacy, sleep quality, and quality of life at 12, 24, and 52 weeks. Successful completion of the proposed study will determine the ideal regimen of Tai Chi and demonstrate that Tai Chi can be a simple, effective, and durable treatment for this therapeutically challenging disorder. We believe that our work will convince the clinical community to implement Tai Chi as a first-line treatment for the millions of individuals with FM.